Culexus are basically a psychic/supernatural dead zones due the very nature of their existence. Hence, such abilities literally just stop working when used against a Culexus, passing through it like it's not even there. A good example is from the novel Nemesis, which also demonstrates what happens to people with psychic powers when exposed to the Culexus:
The guardian, half in and half out of the door, coughed suddenly and blood spluttered from his mouth. He turned, the pain in his skull burning like cold fire, as a figure in glistening black fell the distance from the roof to the courtyard floor. A ring of invisible force radiated out from it, causing a halo of rain to vaporise into mist.
"Kill her!" shouted the warlord, his voice high and filled with terror. "Kill her!"
The psyker took a foot in the spine and Jun shoved him out of the safety of the car, onto his knees. The gull wing door slammed shut and sealed tight.
The Culexus assassin stepped forwards as the guardian got up again, catching sight of the rain rolling down the contours of her skull-helm, dripping from the orbit of the single ruby eye as if she were weeping. The guardian reached inside himself and went deep, past the blazing pain, past the horrific wave of nothingness that threatened to drown him. He found a breath of fire and released it.
The pyrokinetic pulse chugged into existence, streaming from his twitching fingertips. The blast hit the Culexus dead on, and she backed away, shaking her distended steel head; but the tiny flare of hope the guardian experienced died a second later as the fire ebbed, almost as if it had been pulled into the ribbing of the assassin's sinister garb.
He was aware of the car moving forwards in fits and starts, but his attention could not stray from the grinning, angular skull. The sapphire eye-clutch shimmered and the punishing gaze of the weapon known as the animus speculum was turned upon him.
Power, raw and inchoate, sucked in from the fabric of the warp and from the guardian's abortive attack, drawn in like light from the event horizon of a singularity, was now unleashed. A pulse of energy flashed from the psychic cannon and blasted the warlord's bodyguard backward, slamming him into the wall of the courtyard. As he tumbled to the ground, he combusted from within, the fire consuming his flesh and his screams.